Stiffened composite structures are structures that are constructed of composite materials, such as fiber reinforced composite materials, and typically include some form of structural frame that carries a skin. Some modern aircraft fuselages are examples of stiffened composite structures that include a skin operatively coupled to frame members that extend circumferentially around and that are spaced longitudinally along the inside of the fuselage and stringers that extend longitudinally along and that are spaced circumferentially around the inside of the fuselage. Typically, stiffened composite fuselages are constructed utilizing an inner mold line layup mandrel that includes stringer cavities, or forms, that extend longitudinally along the mandrel. Working from the top of the inner mold line layup mandrel, stringers are hand positioned in the stringer cavities. Typical stringers, such as hat-shaped stringers, define cavities themselves, with these cavities needing to be filled with a structure that will result in a flush surface to which the skin may be applied. These filler structures often are referred to as stringer mandrels or stringer bladders. In addition to the stringer mandrels, radius fillers, or noodles, are used at the interfaces between the outer most edges of the stringer mandrels and the stringers to ensure a smooth transition between the outer surfaces of the stringer mandrels and the stringer flanges to which the skin will be applied. Following their placement, the stringer mandrels and noodles are vacuum compacted in batches so that the stringer mandrels and noodles remain in place for subsequent application of the skin. The vacuum compacting process utilizes a gas-impermeable flexible sheet of material that extends across the outer surface of the inner mold line layup mandrel and that is sealed to the inner mold line layup mandrel around the outer perimeter of the stringers and associated stringer mandrels and noodles that are being compacted. A vacuum is then applied between the sheet of material and the inner mold line layup mandrel to compress the stringer mandrels into the stringers. This process is referred to in the aerospace industry as “bagging.” The loading of the stringers and the installation of the stringer mandrels and noodles, as well as the bagging process and the compacting process, are performed on the upper side of the inner mold line layup mandrel, which, for a fuselage of a commercial aircraft, can be very large. Moreover, these processes are all labor and time intensive.